1. Field of the Invention
The present invention relates to an irregularity detection device for a power switch and, more particularly, to a technique pertinent to generation of irregularity alarm report according to if a temperature-dependent on-resistance of a power switch is abnormal.
2. Description of the Related Art
Power switch is a transistor with larger current withstanding capability, less leakage current and better switching characteristics during saturation and cut-off. One of the most prevalent power switches is MOSFET (Metal oxide semiconductor field effect transistor). In view of the foregoing characteristics, power switch is usually applied to power supply equipment, such as switching power supply. With reference to FIG. 7, a conventional switching power supply has a rectification circuit 81, a PFC (Power Factor Correction) circuit 82 and a DC (Direct Current) to DC conversion circuit 83. The PFC circuit 82 has an inductor L1, a diode, a first power switch S1 and a PFC controller located on a DC circuit loop. The DC to DC conversion circuit 83 has a transformer T1, a PWM (pulse width modulation) controller U1 and a second power switch S2. One control terminal of the PWM controller U1 is connected to the second power switch S2. The second power switch S2 is connected to the primary side of the transformer T1.
It can be seen that the conventional switching power supply fully utilizes the power switches as switching elements to adjust the power factor of the switching power supply and perform power conversion. As the conventional PWM controller U1 controls the second power switch S2 by means of a hard switching approach, waveforms associated with switch terminal voltage and current overlap and the overlapping area is not equal to zero. Thus, the second power switch S2 is subject to significant operating characteristics variation when turned on or turn off. In addition to electromagnetic interference, the second power switch is damage-prone because of frequent switching and large operating characteristics variation. To improve the issue arising from the hard switching, a soft switching approach has been developed and brought to the market. The soft switching approach targets at reducing the overlapping area of the voltage and current of a power switch at a switching transient and can even shrink the overlapping area to achieve zero switching loss. Physical implementations about the soft switching approach include zero current switching (ZCS) and zero voltage switching (ZVS).
Although the PWM controller employs the soft switching approach to resolve the issue arising from the hard switching of the second power switch S2, the PFC circuit 82 in the same switching power supply still controls a first power switch S1 by means of the hard switching for the concern of circuit characteristics. Due to frequent hard-switching operation and switching loss, the first power switch S1 becomes failure-prone as a result of abnormal temperature rise. To ensure that power switch in a system can operate stably, irregular working characteristics must be monitored and controlled constantly.